[2] He joined the Italian resistance movement and entered the Action Party (Partito d'Azione, PdA), becoming the secretary together with Ugo La Malfa, Altiero Spineli, Oronzo Reale, and Emilio Lussu.

[5][6][7] When the PdA dissolved in 1947, Foa joined the Italian Socialist Party (Partito Socialista Italiano, PSI), becoming one of its national leaders,[2] and was elected a member of the Chamber of Deputies in 1953 and again in 1958 and 1963.

[2] A leading intellectual of the non-communist left, Foa supported the anti-authoritarian theory of autonomism for the working class, the theory of socialism from below,[8] which would later inspire the foundation of several extra-parliamentary leftist groups of which Foa was to become a member, including the Italian Socialist Party of Proletarian Unity (Partito Socialista Italiano di Unità Proletaria, PSIUP), the Proletarian Unity Party (Partito di Unità Proletaria, PdUP), and Proletarian Democracy (Democrazia Proletaria, DP), of which he was the co-founder after the dissolution of the PSIUP in 1972.

[2] The idea was to give governmental space to extra-parliamentary left-wing political forces in order to bring them into government by diverting them from the revolutionary vision.

Domande di oggi agli inglesi del primo Novecento (1985), a study of English working-class movements at the beginning of the 20th century.

[4] Foa's non-fiction writings in the 2000s continued with Un dialogo (2002, an interview by Carlo Ginzburg), la Memoria è lunga (2003, in collaboration with Federica Montevecchi), and Il linguaggio del tempo (2004).

[2] Despite this, he joined and supported The Olive Tree (L'Ulivo) and the Democrats of the Left (Democratici di Sinistra, DS) led by Romano Prodi and Piero Fassino.

[9] One of his last public appearances took place in 2002 when he joined a rally of the Girotondi [it], a spontaneous anti-Berlusconi civil movement that developed between 2002 and 2003 in several Italian cities, including one held in Rome at Piazza San Giovanni in Laterano.

[2] In 2007, Foa joined the Democratic Party (Partito Democratico, PD), having taken part at its founding congress,[12] supporting Walter Veltroni.

[9] In his last essay, Le parole della politica (2008, in collaboration with Montevecchi), Foa analyzed the degradation of politics and its lexicon, reflecting on the process of emptying the meaning of political language and on the structural reasons that they succumb; in the same year, a contribution by him appeared in Bruno Trentin's volume Lavoro e libertà.

[4] Foa died in Formia on 20 October 2008 after a long illness;[2] the news of his death was given by the then PD leader Walter Veltroni, who described it as "an immense pain".

In 2018, his daughter Anna published the text La Famiglia F., a historical-family narrative in which she reconstructed the life path of her parents.